The invention relates generally to superconducting magnet systems and, more particularly, to a quench protection system of a superconducting magnet system.
In one example, an MR system includes a cold mass comprises a superconducting magnet, a magnet coil support structure, and a helium vessel. Liquid helium contained in the helium vessel provides cooling for the superconducting magnet and maintains the superconducting magnet at a low temperature for superconducting operations, as will be understood by those skilled in the art. The liquid helium maintains the superconducting magnet approximately and/or substantially at the liquid helium temperature of 4.2 Kelvin (K). For thermal isolation, the helium vessel that contains the liquid helium in one example comprises a pressure vessel inside a vacuum vessel.
An MR superconducting magnet typically includes several coils, a set of primary coils that produce a uniform B0 field at the imaging volume, and a set of bucking coils that limit the fringe field of the magnet. These coils are wound with superconductors such as NbTi or Nb3Sn conductors. The magnet is cooled down to liquid helium temperature (4.2 K) so that the conductors are operated at their superconducting state. The heat loads of the magnet, such as that produced by the radiation and conduction from the environment, are removed by either the boil-off of liquid helium in an “open system” or by a 4 K cryocooler in a “closed system”. The magnet is typically placed in a cryostat to minimize its heat loads since the replacement of liquid helium is expensive and since the cooling power of a cryocooler is limited.
One concern in superconducting magnet operation is that of the discontinuance or quenching of superconducting operation, which can produce voltages possibly damaging to the superconducting magnet or which can produce over-heating in the superconducting magnet. Quenching occurs when an energy disturbance, such as from a magnet coil frictional movement, heats a section of superconducting wire, raising its temperature above the critical temperature of superconducting operation. The heated section (normal zone) of wire becomes normal with some electrical resistance. The resulting I2R Joule heating further raises the temperature of the normal zone and increases the normal zone size. An irreversible action called a quench then occurs in which the electromagnetic energy of the superconducting magnet is quickly dumped or converted into thermal energy through the increased Joule heating. In order to provide the required magnetic field homogeneity in the imaging volume for MRI operation the magnet coil is divided into a plurality of sub-coils spaced along and around the axis of the superconducting magnet such that they are not thermally connected. As a result, when one of the superconducting coils quenches, the entire magnetic energy may be dumped into the section of the quenching coil causing a hot spot and possible damage to the coil unless a suitable quench system provides protection which can be accomplished by quenching the other coils.
A conventional superconducting magnet system 2 employs a passive quench protection system 4 as schematically shown in FIG. 1. Superconducting magnet system 2 includes a plurality of superconducting magnet coils 6 electrically connected in series. A superconducting switch 8 is electrically connected in parallel with the plurality of superconducting magnet coils 6. As illustrated, the passive quench protection system 4 includes a plurality of resistive shunts 10 to shunt pairs of superconducting magnet coils 6. However, each resistive shunt 10 may be connected to less or more than the pair of superconducting magnet coils 6 as shown. When a superconducting magnet coil 6 quenches, the resistive shunt 10 connected thereto will allow the current of the superconducting magnet coil 6 to decay faster than the other superconducting magnet coils 6. The voltage developed in a quenching superconducting magnet coil 6 will heat the heaters 9 imbedded in other coils 6 to spread the normal zones, convert the magnetic energy into heat over a large volume of the superconducting coils 6, and thus protect the superconducting magnet system 2.
However, the internal resistive loops formed, in part, by the plurality of resistive shunts 10 may be magnetically coupled with external fields and external field changes such as gradient coil operations in an MRI system or moving metal disturbances. When magnetically coupled to external fields, currents will be induced in the internal resistive loops and resistive shunts 10. These currents can create inhomogeneous magnetic field generated by the plurality of superconducting magnet coils 6. Further, the currents decay over time by the resistance in the internal resistive loops. In an MRI system, an inhomogeneous magnetic field in the imaging volume and a current decay over time may adversely affect the imaging quality of the MRI system. Moreover, the currents in the superconducting magnet coils 6 can be very different during a quench, which may result in a sharp increase in the fringe field of the superconducting magnet to cause fringe field blooming artifacts.
It would therefore be desirable to have a system capable of protecting a superconducting magnet during a quench without the use of internal resistive loops.